Problem: Simplify the following expression: $a = \dfrac{t}{3} - \dfrac{t}{9}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $3$ and $9$ $\lcm(3, 9) = 9$ $ a = \dfrac{3}{3} \cdot \dfrac{t}{3} - \dfrac{1}{1} \cdot \dfrac{t}{9} $ $a = \dfrac{3t}{9} - \dfrac{t}{9}$ $a = \dfrac{3t -t}{9}$ $a = \dfrac{2t}{9}$